1. Field of the Invention
The present invention relates to voltage non-linear resistors consisting essentially of zinc oxide, particularly voltage non-linear resistors for use in lightning arrestors having a gap between a limited current element portion and an insulator which will be referred to as "gapped lightning arrestors" in the present invention.
2. Related Art Statement
Heretofore, resistors consisting essentially of zinc oxide (ZnO) and containing a small amount of metal oxide, such as Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, SiO.sub.2, Co.sub.2 O.sub.3 or MnO.sub.2, etc., as a subsidiary component, having been widely known to exhibit superior non-linear voltage-current characteristic properties, so that they are widely used in lightning arrestors, etc.
Meanwhile, more than half of electrical troubles on overhead power supply or distribution lines arranged on towers at high positions from the ground are occupied by troubles caused by lightnings. If an electric potential of the tower is increased due to hit of a lightning on a power supply or distribution line thereof, the increased electric potential is discharged from the tower via an arc horn, and subsequent trouble current (dynamic current) is shut off by a circuit breaker in a transformer station, so that electric power supply through the power supply or distribution line is stopped. In order to solve the problem, gapless lightning arrestors have hitherto been used. However, though the gapless lightning arrestors have a good response property as well as a superior dynamic current shut-off property, they are always supplied by an electric current, so that they have poor reliability and can not be supplied again with an electric power in case of trouble. Therefore, a gapped lightning arrestor, as schematically shown in FIG. 1, has attracted attentions. Explaining conceptionally, the arrestor is an insulator device 2 with an arc horn 1 having a lightning-arresting function which consists of a limited current element portion 3 and a seriesly connected gap 4. The limited current element portion 3 is composed of seriesly connected zinc oxide elements having a voltage non-linear resistance property and accommodated in an electrically insulative material (insulator pipe) or molded within an electrically insulative material (ethylene-propylene rubber). By this arrangement, the gapped lightning arrestor aims to discharge at the seriesly connected gap 4 an elevated tower potential produced by hit of a lightning on the power supply or distribution line and shuts off the dynamic current in a short period of time by using the voltage non-linear resistance property of the limited current element portion 3 thereby to prevent electric power stoppage caused by actuation of the circuit breaker of the transformer station.
However, when using such gapped arrestor, a gapped arrestor has to be newly inserted between the power supply towers, so that a compact arrestor is required, hence shortening of the length of the limited current element portion is necessary. Also, insulative cooperation of the arc horn 1 is required, and when a power supply line is hit by a lightning a large electric current of the lightning has to be flash connected to the seriesly connected gap 4 so as to prevent a flash connection of the lightning current to the arc horn 1. In order to preferentially flash connect a lightning current to the seriesly connected gap 4 when hit by a lightning, a method can be considered of reducing the number of the voltage non-linear resistor element in the limited current element portion 3 so as to facilitate the flow of the lightning current to the seriesly connected gaps 4. However, large decrease in number of the resistor element allows easily a larger current than conventional to flow, so that the cut-off property of follow current becomes bad. Alternatively, a method can be considered of reducing the distance of the seriesly connected gap 4. However, in such case, a possibility occurs that the seriesly connected gap 4 is flash connected by switching surge discharge current generated by switching of the circuit breaker.